ACT · Elementary Algebra · Substitution and evaluation
Question act-q-00309
Easy Multiple choice No calculator
The question
If f(x) = 4x² − 2, what is the value of f(5)?
- 98
- 99
- 18
- -98
Show answer & worked solution
Answer: 98
Worked solution. Substitute x = 5: f(5) = 4 × (5)² − 2 = 4 × 25 − 2 = 100 − 2 = 98.
Why each wrong choice is wrong. Forgetting to square the input is the dominant error — the linear-substitution distractor catches it.
Test-day tactic. Substitute with parentheses around the input. (5)² always equals 25, regardless of the sign of 5.
About this question type
A value or set of values is given for the variables; substitute and compute. The arithmetic can be tricky when negatives or fractions are involved.
You will see a question shaped like this one on roughly every other official ACT form, typically at an early position in the section, where missing them carries an outsize penalty because the curve assumes everyone in your band gets them right. Treat any miss in this subtopic as a signal to drill the subtopic page before you do another full practice test.